1. Field of the Invention
The present invention relates to a semiconductor device that has an organic semiconductor, a method for manufacturing the same, and an image display device equipped with the semiconductor device, and particularly to a semiconductor device that has an organic semiconductor formed on a resin film, a method for manufacturing the same, and an image display device equipped with the semiconductor device.
2. Description of the Related Art
As the number of information terminals in use increases, needs have been growing for flat panel displays of lighter weight for use in computers. The proliferation of information technologies has also increased the opportunities of handling information, which has previously been conveyed by paper media, in the form of electronic information. This trend has increased the needs for electronic paper or digital paper for mobile display medium that is thin and light weight and can be easily carried (JP 2007-67263A, etc.).
In a flat panel display device in general, a display medium is formed by using elements based on liquid crystal, organic EL (electro-luminescence), electrophoresis or the like. Such a display medium chiefly employs active drive elements (TFT elements) as the picture drive elements, in order to ensure the uniformity of screen brightness, screen refreshing rate and other performance. In a common computer display, for example, the TFT elements are formed on a glass substrate, and liquid crystal or organic EL elements are sealed therein.
As the TFT element, Si semiconductor such as a-Si (amorphous silicon), p-Si (polycrystalline silicon) or the like is mainly used. The Si semiconductor (together with a metal layer as required) is formed in a plurality of layers so as to form source, drain and gate electrodes successively on the substrate, thereby constituting the TFT element.
Formation of the TFT element from the Si semiconductor involves the following two problems.
First, it is necessary to form the layers by repeating the sputtering and other manufacturing processes in a vacuum system that requires a vacuum chamber, thus making the equipment cost and the running cost very expensive. For example, forming each layer requires it to repetitively carry out vacuum vapor deposition, doping, photolithography, development of latent image and other processes, and the element is formed on the substrate through several dozens of processes. The semiconductor that is the critical part of switching operation is also formed by stacking semiconductor layers of a plurality of kinds such as p type and n type. With such a conventional manufacturing method that uses the Si semiconductor, it is difficult to change the production facilities so as to manufacture larger display screens, because it requires significant design change of the production facilities such as the vacuum chamber.
Second, materials to be used are limited to heat resistant ones, and materials such as a resin film that is light in weight and has pliability cannot be used as the substrate.
The process of forming the TFT element from Si includes heating to a temperature as high as 500 to 1,000° C. Therefore, the substrate must be formed from a material that endures such a high temperature, which is practically limited to glass. As a result, when a thin display such as electronic paper or digital paper is made by using TFT elements based on Si semiconductor, use of the glass substrate makes the display heavy and hard without flexibility, thus easily broken when dropped. Therefore, it is difficult to meet the need for a portable and slim display with an image display device constituted by forming TFT elements on a glass substrate.
A semiconductor material that has been vigorously researched in recent years as a promising candidate for solving the problems described above is the organic semiconductor material. The organic semiconductor material is an organic compound that has high charge mobility, and is applicable to an organic laser oscillating element and an organic thin film transistor (organic TFT) as well as the charge transporting material of an organic EL element.
A semiconductor device (organic semiconductor device) based on an organic semiconductor can be made in a process of a relatively low temperature, and therefore imposes a more tolerant requirement of heat resistance on the substrate, so that the TFT elements can be formed on a flexible material such as a transparent resin substrate. An organic semiconductor in the form of solution can also be made by properly modifying the molecular structure. When the organic semiconductor solution is used as an ink in a printing process based on an ink jet method, it is made possible to manufacture the semiconductor device under a condition that does not require a vacuum such as in an inactive gas atmosphere.
Electronics technology based on a printing process makes it possible to carry out the process at a low temperature (get rid of high temperature), mitigation of vacuum process (in addition to the advantage of getting rid of vacuum) and carry out the process without photolithography (get rid of photolithography).
FIG. 15 is a sectional view schematically showing the structure of a semiconductor device (flexible semiconductor device) 1000 that includes an organic semiconductor 130 manufactured by using the printing process. The semiconductor device 1000 a structure including layers (120, 130, 140, 150) formed by printing on a resin substrate (such as PET or PI) 110. In the structure illustrated, a wiring layer 120, an organic semiconductor layer 130, an insulating film 140 and a wiring layer 150 are formed successively on the resin substrate 110. While the specific structure may be altered as required, a source electrode 120s, a drain electrode 120d and a gate electrode 150g are disposed around the organic semiconductor layer 130, thereby forming the organic TFT.
The display that is lighter in weight than the conventional displays and has pliability so as not to break (or is very unlikely to break) when dropped can be made as described above, by forming the TFT elements that drive the display material on the transparent resin substrate.
There are demands for thin displays such as electronic paper or digital paper that are even more compact and lighter in weight. To meet the demands, it is necessary to form the semiconductor elements with higher density in the semiconductor device 1000.
Similarly, there are also strong demands for stationary display devices based on liquid crystal or organic EL that are larger in screen size but smaller in weight and thinner in thickness, and for higher display quality (higher resolution) to be achieved by forming more pixels in a given area. It is necessary to form the semiconductor elements with higher density in the semiconductor device 1000 also for the purpose of meeting these demands.
However, since the semiconductor device 1000 has a structure of flat layers (120, 130, 140, 150) formed one on another on the resin substrate 110, there is a limitation to the improvement of density of semiconductor elements that can be formed.